Valence bond theory 6 orbitals with 6 electrons, benzene

Abstract. Guided by an intuitive choice of approximations which shows remarkable chemical insight into the topic of aromaticity, Huckel mastered the difficult mathematical treatment of a complex molecule like benzene at a very early stage of quantum theory using method 1 (now valence bond theory) and method 2 (now molecular orbital theory). He concluded that methoci 2 is clearly superior to method 1 because the results of this method explain directly the peculiar behaviour of planar molecules with 6 n electrons. 

Benzene is described by valence-bond theory as a resonance hybrid of two equivalent stmctures and is described by molecular orbital theory as a planar, cyclic, conjugated molecule with six tt electrons. According to the Hiickei rule, a molecule must have 4n -H 2 rr electrons, where n = 0, 1, 2, 3, and so on, to be aromatic. Planar, cyclic, conjugated molecules with other numbers of ir electrons are antiaromatic. 

As is well-known, modem valence-bond (VB) theory in its spin-coupled (SC) form (for a recent review, see Ref. 7) provides an alternative description of benzene [8-10] which, in qualitative terms, is no less convincing and is arguably even more intuitive than the MO picture with delocalized orbitals. The six n electrons are accommodated within a single product of six nonorthogonal orbitals, the spins of which are coupled in all five possible ways that lead to an overall six-electron singlet. The simultaneous optimization of the orbitals and of the weights of the five six-electron singlet spin... 

Most organic chemists are familiar with two very different and conflicting descriptions of the 7r-electron system in benzene molecular orbital (MO) theory with delocalized orthogonal orbitals and valence bond (VB) theory with resonance between various canonical structures. An attitude fostered by many text books, especially at the undergraduate level, is that the VB description is much easier to understand and simpler to use, but that MO theory is in some sense more fundamental . 

The concept of a sea of electrons not belonging to any particular atom is reminiscent of the resonance structures covered earlier. The valence electrons in a metal are delocalized just as they are in resonance molecules. The mobile electrons in a bar of sodium are not associated with any particular ion core, just as the electrons in the double bonds of benzene are not associated with any particular atom. To explain this phenomenon in metals, one must apply molecular orbital theory. 

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